Another isostructural system where stripe ordering has been reported is La 2− xSr xCoO 4 (refs 7, 8). The role of this charge stripe instability for the high-temperature superconducting (HTSC) cuprates remains a matter of debate and it is puzzling that both phenomena can coexist, although charge stripes tend to suppress superconductivity 6. As charge stripes act as antiphase domain walls, the magnetic structure appears modulated. These charge stripes are characterized by hole-rich regions running in vertical 4 or diagonal 5 direction within the MO 2 planes ( M=Cu/Ni), thus separating the remaining charge-depleted regions where the antiferromagnetic (AFM) structure of the undoped parent compound recovers. 5) with a quasi-two-dimensional layered structure. 4) and in the isostructural nickelates La 2− xSr xNiO 4+ δ (LSNO) (ref. With the observation of an hour-glass-shaped excitation spectrum in this stripeless insulating cobaltate, we provide experimental evidence that the hour-glass spectrum is neither necessarily connected to charge stripes nor to band structure effects, but instead, probably intimately coupled to frustration and arising chiral or non-collinear magnetic correlations.Ĭharge stripes that have been initially predicted as a combined charge- and spin-density wave phenomenon 1, 2, 3 have been first observed experimentally in Nd-codoped La 2− xSr xCuO 4 (ref. The search for charge stripe ordering reflections yields no evidence for charge stripes in La 1.6Sr 0.4CoO 4, which is supported by our phonon studies. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge-ordered regime of La 2− xSr xCoO 4. Recently, an hour-glass spectrum has been observed in an insulating cobaltate, thus favouring the charge stripe scenario. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. An hour-glass-shaped magnetic excitation spectrum appears to be a universal characteristic of the high-temperature superconducting cuprates.
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